1. Field of the Invention
This invention relates to a phosphating solution, to a phosphating concentrate and to a process for phosphating metal surfaces with aqueous acidic phosphating solutions containing zinc and phosphate ions and at least one organic N-oxide and to their use for pretreating the metal surfaces for subsequent coating, more particularly electrodeposition coating. The process according to the invention may be used for treating surfaces of steel, galvanized or alloy-galvanized steel, aluminium, aluminized or alloy-aluminized steel
2. Discussion of Related Art
The object of phosphating metals is to produce on the metal surface firmly intergrown metal phosphate coatings which improve resistance to corrosion and, in conjunction with paints and other organic coatings, lead to a significant increase in paint adhesion and in resistance to creepage in corrosive environments. Phosphating processes have long been known. Low-zinc phosphating processes where the phosphating solutions have comparatively low contents of zinc ions, for example 0.5 to 2 g/l, are particularly suitable for pretreatment in preparation for painting. A key parameter in low-zinc phosphating baths is the ratio by weight of phosphate ions to zinc ions which is normally of the order of  greater than 12:1 and can assume values of up to 30:1.
It has been found that phosphate layers with distinctly improved corrosion resistance and paint adhesion properties can be formed by using other polyvalent cations than zinc in the phosphating baths. For example, low-zinc processes where, for example, 0.5 to 1.5 g/l manganese ions and, for example, 0.3 to 2.0 g/l nickel ions are added are widely used as so-called trication processes for preparing metal surfaces for painting, for example for the cathodic electrodeposition painting of car bodies.
DE-A-40 13 483 describes phosphating processes with which it is possible to obtain corrosion resistance properties comparable with those achieved by the trication process. These processes use copper in low concentrations (0.001 to 0.03 g/l) instead of nickel. Oxygen and/or other similarly acting oxidizing agents are used to oxidize the divalent iron formed during the pickling of steel surfaces into the trivalent stage. The other similarly acting oxidizing agents mentioned include nitrite, chlorate, bromate, peroxy compounds and organic nitro compounds, such as nitrobenzene sulfonate. German patent application DE 42 10 513 modifies this process by adding hydroxylamine, salts or complexes thereof in a quantity of 0.5 to 5 g/l hydroxylamine to modify the morphology of the phosphate crystals formed.
The use of hydroxylamine and/or hydroxylamine compounds for influencing the form of the phosphate crystals is known from a number of published patent applications. According to EP-A-315 059, a particular effect of using hydroxylamine in phosphating baths is that the phosphate crystals are formed in a desirable columnar or nodal form on steel, even when the concentration of zinc in the phosphating bath exceeds the normal range for low-zinc processes.
Hydroxylamine has the major process-related advantage that it generally does not decompose on its own in the phosphating bath or in phosphating concentrates. Accordingly, it is possible to produce phosphating bath concentrates and regenerating solutions for phosphating baths which directly contain the necessary quantities of accelerator. There is, therefore, no need for complicated subsequent addition of the accelerator in a separate step, as is necessary, for example, where nitrite or hydrogen peroxide is used as the accelerator. However, if the phosphating solution contains copper ions, which is a current trend in the art, hydroxylamine gradually decomposes under the catalytic influence of those ions. In this case, the accelerator has to be separately added to the phosphating bath in large quantifies. Accordingly, there is a need for new accelerators which can be incorporated similarly to hydroxylamine in phosphating baths, phosphating bath concentrates and regenerating solutions without decomposing after a short time. The phosphating baths, concentrates and regenerating solutions are expected to have this property even in the presence of copper ions.
The problem addressed by the present invention was to provide a phosphating process which would have the advantages of hydroxylamine-accelerated processes without any of their disadvantages in regard to decomposition in the presence of copper ions. The phosphating process would lend itself to application by spraying, spraying/dipping or dipping.
Accordingly, the present invention relates to an acidic aqueous phosphating solution containing
0.2 to 3 g/l zinc ions,
3 to 50 g/l phosphate ions expressed as PO43xe2x88x92 and
accelerators,
characterized in that the solution contains
0.05 to 4 g/l of an organic N-oxide as accelerator.
Preferred organic N-oxides are those which have a saturated, unsaturated or aromatic 5- or 6-membered ring system and in which the N-atom of the N-oxide is part of that ring system. Examples of such compounds are the N-oxides of substituted or unsubstituted pyrroles, imidazoles, oxazoles, pyridines, pyrimidines, pyrazines, oxazines or hydrogenation products thereof which have saturated or partly unsaturated rings. It is particularly preferred to use N-oxides of substituted or unsubstituted pyridines and morpholines such as, in particular, pyridine-N-oxide, 2-methyl pyridine-N-oxide 4-methyl pyridine-N-oxide, morpholine-N-oxide and N-methyl morpholine-N-oxide. The last of these N-oxides is particularly preferred.
The phosphating solution may contain one or more of these N-oxides. The total concentration of the N-oxides in the phosphating solution is preferably in the range from 0.1 to 3 g/l and more preferably in the range from 0.3 to 2 g/l. With lower concentrations, the accelerating effect diminishes; higher concentrations are harmless, but do not afford any technical advantage and are therefore uneconomical.
Besides zinc ions, phosphating baths generally contain sodium, potassium and/or ammonium ions for adjusting the free acid. The concept of the free acid is familiar to the expert on phosphating. The method selected in this specification to determine the free acid and the total acid is described in the Examples. Free acid values of 0 to 1.5 points and total acid values of about 15 to about 35 points are in the technically normal range and are suitable for the purposes of the present invention.
The zinc contents are preferably in the range from 0.4 to 2 g/l and more preferably in the range from 0.5 to 1.5 g/l which is normal for low-zinc processes. The ratio by weight of phosphate ions to zinc ions in the phosphating baths may vary within wide limits providing it is in the range from 3.7 to 30:1. A ratio by weight of 10 to 20:1 is particularly preferred.
It has been found in practice that, where an organic N-oxide is used as accelerator in accordance with the present invention, it is advisable to use relatively highly concentrated activating solutions for the activation step normally preceding the phosphating process. However, if the organic N-oxide is supplemented by a suitable co-accelerator, activation may be carried out in the usual way.
Accordingly, a phosphating solution according to the invention additionally containing about 0.3 to about 4 g/l chlorate ions is preferably used. The chlorate concentration is preferably in the range from 1 to 3 g/l. Instead of or together with the chlorate ions, the phosphating solution may contain one or more of the following accelerators in addition to the organic N-oxide:
0.003 to 0.03 and preferably 0.005 to 0.015 g/l hydrogen peroxide in free or bound form,
0.2 to 1 and preferably 0.25 to 0.5 g/l nitroguanidine,
0.15 to 0.8 and preferably 0.2 to 0.5 g/l m-nitrobenzene sulfonate ions.
Hydrogen peroxide may be added as such to the phosphating solution. However, it may also be used in bound form in the form of compounds which form or eliminate hydrogen peroxide in the phosphating bath. Examples of such compounds are perborates, percarbonates, salts of peroxo acids such as, for example, peroxodisulfate or peroxides such as, for example, sodium or potassium peroxide.
Chlorate ions and/or m-nitrobenzene sulfonate ions are preferably used in the form of their water-soluble salts, for example their alkali metal salts.
Phosphating solutions containing other monovalent or divalent metal ions, which have been found by experience to have a favorable effect on the paint adhesion and corrosion prevention of the phosphate layers produced, are preferably used in the phosphating process according to the invention. In a preferred embodiment, therefore, the phosphating solution according to the invention additionally contains one or more of the following cations:
0.1 to 4 g/l manganese(II),
0.2 to 2.5 g/l magnesium(II),
0.2 to 2.5 g/l calcium(II),
0.002 to 0.2 g/l copper(II),
0.1 to 2 g/l cobalt(II).
If desired, the phosphating solutions may additionally contain nickel ions. However, phosphating baths which have minimal contents of nickel ions or, if desired, may even be nickel-free are preferred for health reasons and ecological reasons.
In one preferred embodiment, for example, the phosphating solution according to the invention contains 0.1 to 4 g/l manganese ions and 0.002 to 0.2 g/l copper ions and no more than 0.05 g/l and, in particular, no more than 0.001 g/l nickel ions as additional cations besides zinc ions. However, if it is desired to stay with the conventional trication technology, phosphating baths according to the invention containing 0.1 to 4 g/l, manganese ions and in addition 0.1 to 2.5 g/l nickel ions besides zinc ions may be used. In principle, the form in which the cations are introduced into the phosphating baths is of no relevance. However, it is particularly appropriate to use oxides and/or carbonates as the cation source.
In the phosphating of zinc-containing surfaces, it has proved to be favorable to limit the nitrate content of the phosphating bath to at most 0.5 g/l. This suppresses the problem of so-called fisheye formation and improves protection against corrosion, particularly where nickel-free phosphating baths are used. Nitrate-free phosphating baths are particularly preferred.
In the case of phosphating baths intended to be suitable for various substrates, it has become standard practice to add free and/or complexed fluoride in quantities of up to 2.5 g/l total fluoride, including up to 750 mg/l free fluoride, expressed as Fxe2x88x92. The presence of fluoride in such quantities is also of advantage for the phosphating baths according to the invention. In the absence of fluoride, the aluminium content of the bath should not exceed 3 mg/l. In the presence of fluoride, higher Al contents are tolerated as a result of complexing providing the concentration of the non-complexed Al does not exceed 3 mg/l.
In principle, phosphating baths may be prepared by dissolving the individual components in water in situ in the required concentration range. In practice, however, it is normal to use concentrates which contain the individual constituents in the required quantities and from which the ready-to-use phosphating bath is prepared in situ by dilution with water or which are added as a regenerating solution to a working phosphating bath in order to compensate for the consumption of active components. However, phosphating concentrates such as these are adjusted to a highly acidic pH for stabilization. After dilution with water, therefore, the pH value and/or the free acid has to be neutralized fairly often to the required range. Alkaline substances such as, for example, sodium hydroxide or sodium carbonate or basic salts or hydroxides of Ca, Mg, Zn are added for this purpose.
Accordingly, the present invention also relates to an aqueous concentrate which, after dilution with water by a factor of 10 to 100 and optionally after pH adjustment to a working range of 2.5 to 3.6, gives a phosphating solution of the type claimed in one or more of claims 1 to 13.
The present invention also relates to a process for phosphating metal surfaces of steel, galvanized or alloy-galvanized steel and/or of aluminium. The materials mentioned may also be present alongside one another, as is increasingly the case in car manufacture. The metal surfaces are contacted with the phosphating solution according to the invention by spraying or dipping or by a combination thereof. The temperature of the phosphating solution is preferably in the range from about 40 to about 60xc2x0 C.
The phosphating process may be used for phosphating steel or galvanized steel strip in strip mills. The phosphating times are of the order of about 3 to about 20 seconds. However, the process may be used in particular in the automotive industry where treatment times of 1 to 8 minutes are normal. It is particularly intended for the treatment of the above-mentioned metal surfaces in preparation for painting, more particularly cathodic electrodeposition painting. The phosphating process may be regarded as part of the normal pretreatment chain. Within this chain, phosphating is generally preceded by cleaning/degreasing, intermediate rinsing and activation steps, activation normally being carried out with titanium phosphate activators. The phosphating treatment according to the invention may be followed, optionally after rinsing, by a passivating aftertreatment. Chromic acid treatment baths are widely used for such an aftertreatment. However, in the interests of safety at work and environmental protection and for reasons of waste disposal, there is a tendency to replace these chromium-containing passivating baths by chromium-free treatment baths. Pure inorganic baths, more particularly based on zirconium compounds, and even organic baths, for example based on poly(vinylphenols), are known for this purpose. Where phosphating solutions containing neither nickel nor copper ions are used, a distinct improvement in corrosion prevention can be obtained by adding copper or silver ions to the baths for the passivating aftertreatment. For example, passivating after-rinse solutions which contain 0.001 to 10 g/l copper ions and which if desired may be free from other passivating components may be used. In general, an intermediate rinse with deionized water is carried out between this after-passivation step and the subsequent electrodeposition painting process.